1. Field of the Invention
The present invention relates to microstructure devices, and more particularly to isolating stress sensitive microstructure devices from packaging stress and the like.
2. Description of Related Art
A variety of devices are known in the art for isolating semiconductor dies from packaging stress and the like. Packaging stress or mounting stress is the stress imparted on a semiconductor die by the package to which it is mounted. This can arise due to the semiconductor die having a different coefficient of thermal expansion from the packaging to which it is mounted and/or from the adhesive mounting the die to the package. In such cases, a change in temperature can cause a stress/strain on the semiconductor die, and depending on the function of the die, this stress/strain can impair performance. Packaging stress can also be caused by mechanical mounting effects from how a die is mounted to the package and how the package itself is mounted in its surroundings.
In one example of packaging stress, traditional piezo resistive MEMS pressure sensor packages are designed to sense the stress on a diaphragm due to an applied pressure. It is therefore important that the only stress that the piezo resistors experience is due to the applied pressure and not to packaging stress. In such sensor packages, wherein the MEMS die is typically mounted directly to a metallic package, there can be significant packaging stress due to mechanical mounting stress and thermal expansion stress as explained above. Such sensor packages are inexpensive, but the packaging stress on the diaphragm makes pressure measurement problematic in terms of accuracy.
Another example of how packaging stress can undermine proper functioning of a microstructure device is in MEMS accelerometers where small deflections of a proof mass are measured to determine acceleration. Packaging stress imparted on such devices can induce undesirable deflections between the proof mass and the sensing structures in the device, which can give rise to false readings of acceleration. Other exemplary devices that are negatively affected by packaging stress include MEMS gyros for sensing angular movement, temperature sensors, and the like.
Typical approaches to minimize adverse packaging stress and strain include using a complaint adherence such as soft or elastomeric adhesives. This approach is fairly inexpensive and easy to manufacture and provides partial stress relief, but has certain disadvantages including processing (i.e., curing), out-gassing, inconsistent mechanical properties over temperature, and potential media incompatibility. Other approaches include fixed mounting methods such as fusion, frit, solder, braze, anodic and eutectic attachment. These can provide advantageous media compatibility, more consistent mechanical properties, and can be more robust compared to other techniques, but can cost more, can require specialized processing equipment and processes as well as higher temperature processing, and can be a potential stress inducer. Still other approaches include MEMS structure additions such as springs and mounting pedestal geometries. These techniques offer potential advantages such as springs being integral with the MEMS structure, additional stress relief may not be required, and smaller size potential. However, these techniques have disadvantages including higher development cost compared to other techniques, and mechanical resonance issues that need to be addressed. Often, multiple approaches such as those above are utilized together to address packaging stress.
For example, in some traditional MEMS pressure sensor packages, packaging stress mitigation was achieved by thickening the topping and backing wafers enclosing the diaphragm, adding a high-aspect ratio pedestal between the package and the MEMS die, and using a large, custom package to house it all. These measures have been found to provide an order of magnitude increase in accuracy in measurements made with the sensor packages so configured. However, the stress mitigation features add to the cost and size of the sensor packages.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for microstructure device packages that allow for improvements in packaging stress mitigation. There also remains a need in the art for such devices that are easy to make and use. The present invention provides a solution for these problems.